1. Field of the Invention
The present invention relates to a zoom lens system of a four-lens-group arrangement for a compact camera, and in particular, relates to increasing the zoom ratio thereof.
2. Description of the Prior Art
Unlike a zoom lens system of a single lens reflex (SLR) camera which requires space to accommodate a quick-return mirror behind the photographing lens system, a zoom lens system of a compact camera does not require a long back focal distance. Therefore a compact camera generally employs a telephoto-type zoom lens system having a positive front lens group and a negative rear lens group in this order from the object, while a SLR camera employs a retrofocus-type zoom lens system having a negative front lens group and a positive rear lens group in this order from the object.
In such a telephoto-type zoom lens system for a compact camera, a zoom ratio of about 3 can be attained by a zoom lens system of a two or three-lens-group arrangement. However, if the zoom ratio is increased up to 4 or 5, even a zoom lens system of a three-lens-group arrangement has to increase the traveling distance thereof, and the size of the zoom lens system becomes larger.
Furthermore, in such a zoom lens system of a three-lens-group arrangement, if an attempt is made to reduce the traveling distance thereof by making the power of each lens group stronger, the number of lens elements are inevitably increased in order to correct aberrations which occur in each lens group. Consequently, the thickness of each lens group is increased, and therefore the overall length of the zoom lens system is increased. Moreover, the overall length of the zoom lens system at the retracted position undesirably increases the thickness of the camera body.
The zoom lens system according to the present invention employs a four-lens-group arrangement which is an improvement of a three-lens-group arrangement. More specifically, the zoom lens system includes a positive lens group, a negative lens group, a positive lens group, and a negative lens group in this order from the object so that a zoom ratio of 5 or more can be attained.
According to an aspect of the present invention, there is provided a zoom lens system including a positive first lens group, a negative second lens group, a positive third lens group, and a negative fourth lens group, in this order from the object. Zooming is performed by moving the positive first through negative fourth lens groups along the optical axis.
The zoom lens system satisfies the following condition:
xe2x88x9212 less than fT/f4G less than xe2x88x9210.5xe2x80x83xe2x80x83(1)
wherein
fT designates the focal length of the entire zoom lens system at the long focal length extremity; and
f4G designates the focal length of the negative fourth lens group.
The zoom lens system preferably satisfies the following condition:
1.1 less than fT23/fW23 less than 1.4xe2x80x83xe2x80x83(2)
wherein
fT23 designates the combined focal length of the negative second lens group and the positive third lens group at the long focal length extremity; and
fW23 designates the combined focal length of the negative second lens group and the positive third lens group at the short focal length extremity.
The zoom lens system can satisfy the following condition:
0.05 less than (dW12xe2x88x92dT12)/fW less than 0.15xe2x80x83xe2x80x83(3)
wherein
dW12 designates the distance between the positive first lens group and the negative second lens group at the short focal length extremity;
dT12 designates the distance between the positive first lens group and the negative second lens group at the long focal length extremity; and
fW designates the focal length of the entire zoom lens system at the short focal length extremity.
The zoom lens system preferably satisfies the following condition:
2.5 less than fT/f1G less than 3.5xe2x80x83xe2x80x83(4)
wherein
f1G designates the focal length of the positive first lens group.
The negative fourth lens group preferably includes cemented lens elements which satisfy the following conditions:
2 less than fT/fc4G less than 4xe2x80x83xe2x80x83(5)
15 less than xcexd4Pxe2x88x92xcexd4Nxe2x80x83xe2x80x83(6)
wherein
fc4G=rc4G/(n4Pxe2x88x92n4N);
rc4G designates the radius of curvature of the bonding surface of the cemented lens elements in the negative fourth lens group;
n4P designates the refractive index of a positive lens element of the cemented lens elements;
n4N designates the refractive index of a negative lens element of the cemented lens elements;
xcexd4P designates the Abbe number of the positive lens element of the cemented lens elements; and
xcexd4N designates the Abbe number of the negative lens element of the cemented lens elements.
The positive first lens group preferably includes cemented lens elements which satisfy the following conditions:
0.2 less than fT/fc1G less than 0.4xe2x80x83xe2x80x83(7)
5 less than xcexd1Pxe2x88x92xcexd1Nxe2x80x83xe2x80x83(8)
wherein
fc1G=rc1G/(n1Pxe2x88x92n1N);
rc1G designates the radius of curvature of the bonding surface of the cemented lens elements in the positive first lens group;
n1P designates the refractive index of a positive lens element of the cemented lens elements;
n1N designates the refractive index of a negative lens element of the cemented lens elements;
xcexd1P designates the Abbe number of the positive lens element of the cemented lens elements; and
xcexd1N designates the Abbe number of the negative lens element of the cemented lens elements.
The zoom lens system can satisfy the following condition:
5.2 less than fT/fWxe2x80x83xe2x80x83(9)
The positive third lens group preferably includes a lens element having at least one aspherical surface which satisfies the following condition:
xe2x88x9230 less than xcex94IASP less than xe2x88x9210xe2x80x83xe2x80x83(10)
wherein
xcex94IASP designates the amount of change of the spherical aberration coefficient due to the aspherical surface in the positive third lens group under the condition that the focal length at the short focal length extremity is normalized to 1.0.
The negative fourth lens group preferably includes a lens element having at least one aspherical surface which satisfies the following condition:
0 less than xcex94VASP less than 0.6xe2x80x83xe2x80x83(11)
wherein
xcex94VASP designates the amount of change of the distortion coefficient due to the aspherical surface in the negative fourth lens group under the condition that the focal length at the short focal length extremity is normalized to 1.0.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2002-175394 (filed on Jun. 17, 2002) which is expressly incorporated herein in its entirety.